1. Field of the Invention
The present invention relates to an apparatus for the automatic sensing and positioning of a registration mark, indicia, or line target having finite contrast, either optically, magnetically, acoustically or electromagnetically relative to adjacent background utilizing a pseudo match of sensor width to registration mark width.
2. Description of the Prior Art
Mark sensing and scanning apparatus are disclosed in U.S. Pat. No. 3,335,281 to S. P. Willits. Such apparatus utilized a single sensor, having a particular physical width and length, in a cyclic scanning system. FIGS. 1, 4 and 5 in the prior patent show the waveform as generated by the photosensor scanning a given mark. The disclosure in this patent provides an insight into the capabilities of a system utilizing a sensing apparatus which automatically determines and adjusts the scan amplitude relative to the position error of the registration mark. Among these capabilities are a system for obtaining automatic scan amplitude change; higher positional accuracy due to higher loop gain with reduced scan amplitude of the scanning sensor; and demodulation of the sensor's output after amplification in a symmetrical, amplitude saturated amplifier.
While the apparatus suggested by this prior art patent solved many of the apparent problems present at that time, associated with "mark" positioning, and in most instances, provided excellent results for a wide variety of target width and target contrast ratios, it had definite limitations and restrictions in both loop gain characteristics and positional accuracy required by today's wide varieties of target and by the requirement to position to a very high degree of accuracy.
The concept of a "pitch match" sensor array for counting stacks of material having essentially the same thickness or pitch for each of the tightly packed sheets, is disclosed in: the Mohan & Willits U.S. Pat. No. Re. 27,869, where the concept of "pitch match" is described; the Mohan et al. U.S. Pat. No. 4,373,135, where the concept of a pseudo pitch match sensor array is described; and the Mohan et al. U.S. Pat. No. 3,663,803 where an apparatus for specific mark code decoding of a coded panel having relative linear velocity to a pitch matched sensor array is described.
The target sensor characteristics common to all of these pitch match devices are the cyclical repetitive nature of the data base presented to the sensing array and the signal enhancement gained by the technique of achieving the equivalent of spatial filtering by adapting either a single sensor into a pseudo pitch match sensor array or by physically matching a multiple sensor scanning array into a desired spatial pitch match array.
The positioning apparatus and method of the present invention are utilized in a device which automatically senses an indicia on a workpiece and positions the workpiece to bring the indicia into registration by diminishing registration error to near zero. More particularly, the device and method are adapted to sense preferably elongate indicia of varying widths utilizing a positioning apparatus with one or more sensing heads which automatically adapts its sensing modes utilizing comparisons of delayed time data with real time data to a variety of indicia widths and degrees of registration error to rapidly bring the indicia into accurate registration.
As explained above, automatic positioning devices for workpieces have been in existence almost since the beginning of the manufacture of controls. Their goal is to rapidly and accurately position a workpiece for some further operation. Typically, the workpiece to be positioned will bear one or more indicia which will allow the positioning device to accurately sense the indicia and then accurately position the workpiece or bring the workpiece into registration. Typically, the speed of positioning must be traded off against the accuracy of the final registration position for fixed control parameters and fixed inertia associated with the workpiece and movable elements of the positioning device.
The apparatus of the present invention senses an indicia and then moves the workpiece to bring the indicia into registration. The assignee of this application, Spartanics Ltd. of Rolling Meadows, Ill., owns a number of U.S. Patents disclosing apparatus for sensing an indicia or positioning a workpiece bearing an indicia or, in a somewhat unrelated field for sensing and counting sheets in a stack. Among these patents are the following, the disclosure of each of which is hereby incorporated by reference:
______________________________________ U.S. Pat. No. Patentee ______________________________________ 3,335,281 Willits Re. 27,869 Willits et al. 3,790,759 Mohan et al. 3,813,523 Mohan et al. 4,373,135 Mohan et al. 4,542,470 Mohan et al. ______________________________________
The significant teachings of these prior patents to the apparatus and method of the present invention are set forth below:
U.S. Pat. No. 3,335,281 to Willits is for a SYMMETRICALLY SATURATED POSITION SERVO CONTROL WITH DUAL AMPLITUDE OPTICAL OFFSET SCANNING. The control of this patent employs a single scanning head to sense each indicia. Further, by employing amplifiers in a saturated mode which does not affect the harmonic content and phase relationship of the sensing signal, the harmonic content and phase relationship being position dependent, exceptional stability over wide variances in loop gain is achieved. Until an indicia is sensed and moved near registration, the scan amplitude of the scanning head is large. Thereafter, the scan amplitude is reduced to provide for more accurate registration. While this control solved many problems in this field twenty-odd years ago, it is relatively intolerant of wide variations in indicia characteristics and is somewhat inaccurate in positioning by today's standards.
U.S. Pat. No. Re. 27,869 to Willits and the present inventor, William Mohan, is for a PITCH MATCHING DETECTING AND COUNTING SYSTEM utilized in counting sheets in a stack. While counting is a far different activity from positioning, this system does employ a sensing head which must distinguish between each of several objects to be counted by distinguishing each object from a near negligible interstitial boundary between the object and adjacent objects. Here the concept of matching a pair of sensors to the width of the object to be counted (pitch matching) was employed to spatially filter unwanted harmonics with their significant contribution to error. When a pair of PM sensors are employed, the phase shift between their respective signal combinations may be employed as an error signal, indicating the degree to which pitch match is achieved. A single sensor oscillating between two limits of oscillation may, if suitably manipulated, provide signals equivalent to that provided by a sensor pair.
U.S. Pat. No. 4,373,135 to Mohan, Willits and Kleemann is for a PITCH MATCH DETECTING AND COUNTING SYSTEM that is an improvement to the system disclosed in U.S. Pat. No. Re. 27,869. This patent discloses the employment of a tapped delay line to synthesized from a single sensor, the equivalent of one or more sensors whose signal outputs are the equivalent of those generated by a sensor array, pitch matched spatially or optically. A sensor signal generated in counting a stack of similar objects or elements is cyclic as the sensed area passes over a plurality of objects which are substantially identical. When a pair of sensors, or sensor arrays, are employed, the signals produced in each by a given, particular element is substantially identical to the other but displaced in time, or phase shifted, as a result of the spatial displacement of the sensors one from the other.
According to the teachings of the present invention, once a single sensor signal is generated (given known parameters) it may be electrically stored and employed to automatically produce a pseudo sensor whose signal is pitch matched to the equivalent of a pitch match system, although the real sensor effective width may be very narrow in comparison with the counted object width or pitch.